1. Field of the Invention
The present invention relates to a method for producing an L-amino acid using a bacterium, and more particularly, to a method for producing an L-amino acid such as L-lysine, L-threonine, and L-tryptophan. L-lysine, L-threonine, and L-tryptophan are industrially useful as additives for animal feeds, components of health foods, amino acid infusions, and the like.
2. Brief Description of the Related Art
As methods for producing a target substance such as an L-amino acid by a fermentation method using a bacterium, there are known a method using a wild-type bacterium (wild-type strain), a method using an auxotrophic strain induced from a wild-type strain, a method using a metabolic control mutant induced from a wild-type strain as various drug-resistant mutants, and a method using a strain having properties of both the auxotrophic strain and the metabolic control mutant.
In recent years, fermentative production of a target substance is performed using a recombinant DNA technology. For example, improvement of L-amino acid productivity of a bacterium has been achieved by enhancing the expression of a gene encoding an L-amino acid biosynthetic enzyme (U.S. Pat. No. 5,168,056 and U.S. Pat. No. 5,776,736) or by enhancing the influx of a carbon source into an L-amino acid biosynthetic system (U.S. Pat. No. 5,906,925).
Glutamate decarboxylase (GAD) from Escherichia coli is an enzyme that produces γ-aminobutyric acid (GABA) from L-glutamic acid by decarboxylation reaction (Methods Enzymol. 1985. 113: 11-16. Fonda, M. L. L-Glutamate decarboxylase from bacteria.), and is known to play an important role in the resistance to an acid (J. Bacteriol. 1999. 181(11): 3525-3535. Castanie-Cornet, M. P., Penfound, T. A., Smith, D., Elliott, J. F., and Foster, J. W. Control of acid resistance in Escherichia coli.). Two kinds of GADs have been discovered in Escherichia coli, and one is encoded by gadA gene, and the other is encoded by gadB gene (J. Bacteriol. 1992. 174(18): 5820-5826. Smith, D. K., Kassam, T., Singh, B., and Elliott, J. F. Escherichia coli has two homologous glutamate decarboxylase genes that map to distinct loci.).
Meanwhile, the gadC gene is located downstream of gadB gene and forms an operon with the gadB gene, and is presumed to encode a glutamic acid/GABA antiporter (J. Bacteriol. 1996. 178(13): 3978-3981. Hersh, B. M., Farooq, F. T., Barstad, D. N., Blankenhorn, D. L., and Slonczewski, J. L. A glutamate-dependent acid resistance gene in Escherichia coli.). It has been disclosed that elimination of gadB is effective for production of an L-amino acid such as L-glutamic acid, L-proline, or L-leucine (RU Patent 2,264,457). However, there have been no reports of the production of an L-amino acid using a bacterium modified to enhance genes encoding GAD (gadA and gadB) or a gene encoding a transporter of GABA (gadC).